PROJECT SUMMARY/ABSTRACT Aneuploidy - the wrong number of chromosomes in an individual - is the leading cause of birth defects in humans. It results from errors in the segregation of homologous chromosomes (homologs) during gametogenesis. The proper segregation is ensured by meiotic recombination. It begins with the introduction of DNA double stranded breaks (DSBs) followed by their repair using the intact DNA of a homologous chromosome as a template. This leads to a temporal association of the homologs stabilized by crossing-overs (COs). Such an arrangement into pairs ensures orderly segregation of the homologous chromosomes to the opposite poles of dividing nuclei so that each gamete receives one homolog of each pair. The homologs that fail to pair segregate randomly, and have a 50% chance to go into the same daughter cell. Along with the lack of COs, positional effects of CO placement also contribute to aneuploidy. Spatial distribution of recombination events is controlled at different levels and defining the mechanisms of this regulation is necessary to understand why some of the events escape this control. Our long-term goal is to elucidate the mechanisms behind faulty meiotic recombination resulting in aneuploidy in mammals. In this study we will take a genome-wide approach to define the mechanisms of CO placement in the mouse. (i) By cytological evaluation of the mouse meiotic chromosomes we will determine whether placement of meiotic DSBs is random or displays interference, i.e. the formation of one DSB suppresses the formation of a second one in adjacent regions. This will help to clarify the mechanisms involved in imposition of CO interference. (ii) Using chromatin immunoprecipitation followed by direct high-throughput sequencing we will map the regions of mouse genome where meiotic DSBs tend to occur (hotspots of meiotic DSBs). Recombination is not evenly distributed throughout the genome and defining the particular features associated with recombination hotspots will provide the cues to the mechanism behind their formation. (iii) We will map the hotspots of meiotic COs using the similar approach. Comparative analysis of these two maps will help to elucidate the pathways leading to CO formation, and the mechanisms involved in CO/NCO designation. Overall, the results from our studies will illuminate several important aspects of the mechanisms involved in CO placement, and will also create new avenues for future research in delineating the mechanism of meiotic recombination control. Mutations that reduce or abolish recombination are invariably associated with meiotic arrest or chromosome segregation errors leading to infertility or aneuploidy. Understanding the forces driving recombination is necessary before preventive measures and therapeutic approaches can be developed.